fix page error viewing external tool assignment with blank URL
fix page error viewing external tool assignment with blank URL

test plan:
- on the assignments page, click + to add an assignment,
  set the type to External Tool, enter a name, and click Save.
- copy to another course
- try to view the assignment in the destination course.
  you should not see a 500 page error. (you may see "Couldn't find
  valid settings for this link." pop up instead.)

fixes CNVS-18051

Change-Id: I89873e6b91814013206166b7a43d61c41cb2ae6f
Reviewed-on: https://gerrit.instructure.com/69950
Tested-by: Jenkins
Reviewed-by: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: Jahnavi Yetukuri <ed2b2672ccc2e59e56c0615df7f01b579202e014@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

roxolan/canvas-lms,matematikk-mooc/canvas-lms,fronteerio/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,redconfetti/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,roxolan/canvas-lms,roxolan/canvas-lms,HotChalk/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,redconfetti/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,HotChalk/canvas-lms,matematikk-mooc/canvas-lms,HotChalk/canvas-lms,redconfetti/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,HotChalk/canvas-lms,roxolan/canvas-lms,matematikk-mooc/canvas-lms,grahamb/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,fronteerio/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,redconfetti/canvas-lms,dgynn/canvas-lms,instructure/canvas-lms,fronteerio/canvas-lms,instructure/canvas-lms,dgynn/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,SwinburneOnline/canvas-lms,dgynn/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,dgynn/canvas-lms,instructure/canvas-lms,fronteerio/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms